I. AIR Synthetase
The AIR synthetase is an enzymatic step in the de novo purine biosynthesis pathway, which leads to the synthesis of the purine nucleotides IMP, AMP and GMP. De novo purine biosynthesis plays a central role in the nitrogen assimilation pathway and is conserved among bacteria, yeast, Drosophila and mammals (Schnorr et al. (1994) The Plant Journal, 6: 113-121). The AIR synthetase enzymatic activity corresponds to the fifth step in the pathway and catalyzes the conversion of 5'-phosphoribosyl-N-formylglycinamidine (FGAM) to 5'-phosphoribosyl-5-aminoimidazole (AIR). In E. coli, this step is carried out by a protein encoded by the purM gene. Recently, an Arabidopsis c-DNA encoding an enzyme having AIR synthetase activity has been cloned and its sequence has been determined (Senecoff and Meagher (1993) Plant Physiol. 102: 387-399; Schnorr et al. (1994) The Plant Journal, 6: 113-121).
II. Herbicide Discovery
The use of herbicides to control undesirable vegetation such as weeds in crop fields has become almost a universal practice. The herbicide market exceeds 15 billion dollars annually. Despite this extensive use, weed control remains a significant and costly problem for farmers.
Effective use of herbicides requires sound management. For instance, the time and method of application and stage of weed plant development are critical to getting good weed control with herbicides. Since various weed species are resistant to herbicides, the production of effective new herbicides becomes increasingly important. Novel herbicides can now be discovered using high-throughput screens that implement recombinant DNA technology. Metabolic enzymes found to be essential to plant growth and development can be recombinantly produced though standard molecular biological techniques and utilized as herbicide targets in screens for novel inhibitors of the enzymes' activity. The novel inhibitors discovered through such screens may then be used as herbicides to control undesirable vegetation.
III. Herbicide Tolerant Plants
Herbicides that exhibit greater potency, broader weed spectrum, and more rapid degradation in soil can also, unfortunately, have greater crop phytotoxicity. One solution applied to this problem has been to develop crops that are resistant or tolerant to herbicides. Crop hybrids or varieties tolerant to the herbicides allow for the use of the herbicides to kill weeds without attendant risk of damage to the crop. Development of tolerance can allow application of a herbicide to a crop where its use was previously precluded or limited (e.g. to pre-emergence use) due to sensitivity of the crop to the herbicide. For example, U.S. Pat. No. 4,761,373 to Anderson et al. is directed to plants resistant to various imidazolinone or sulfonamide herbicides. The resistance is conferred by an altered acetohydroxyacid synthase (AHAS) enzyme. U.S. Pat. No. 4,975,374 to Goodman et al. relates to plant cells and plants containing a gene encoding a mutant glutamine synthetase (GS) resistant to inhibition by herbicides that were known to inhibit GS, e.g. phosphinothricin and methionine sulfoximine. U.S. Pat. No. 5,013,659 to Bedbrook et al. is directed to plants expressing a mutant acetolactate synthase that renders the plants resistant to inhibition by sulfonylurea herbicides. U.S. Pat. No. 5,162,602 to Somers et al. discloses plants tolerant to inhibition by cyclohexanedione and aryloxyphenoxypropanoic acid herbicides. The tolerance is conferred by an altered acetyl coenzyme A carboxylase (ACCase).